Allow `__debug__` to be set at runtime

[elis.byberi]

We could allow it to be set to False at runtime, but setting it to True wouldn’t work because the bytecode might not include debugging information (the source code could be missing), and the necessary debugging libraries might not be available on the system (in a production environment).

That said, this change would introduce backward incompatibility.

I personally use -O and -OO in production.

Does anyone use `-O` or `-OO` or `PYTHONOPTIMIZE=1` in their deployments?

I expect the answer to the question in the subject for most is “No”… In that case, you are not who I’m interested in hearing from. It’d be good to hear from CPython users who actually use -O, -OO, or PYTHONOPTIMIZE=1 to have Python elide assert statements (performance?) or (-OO) omit docstrings from bytecode (memory savings?). Some idea of: Why you do it? How and if you measure the benefit of doing so and how often do you re-measure? Similarly, if you own PyPI packages, do you routinely test…